Add arrow to the same node in latex tikz

Question

I'm trying to create the schema in tikz latex. I need to have arrows between nodes and arrows to the same node. The arrow to the same node has to be outside the node. It has to start on the top right of the box and end on the top left of the box.

How to add a bent arrow to the box as shown in the picture. I would love to have the same arrow as shown in the screenshot.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[
node distance=1.5cm and 1cm,
ar/.style={->,>=latex},
middle_node/.style={
  draw,
  text width=1.5cm,
  minimum height=0.75cm,
  align=center
  },
end_node/.style={
  draw,
  text width=1cm,
  minimum height=0.55cm,
  align=center
  }
]

  % nodes
  \node[end_node] (start) {\textbf{start}};
  \node[middle_node,right=of start] (first_step) {a};
  \node[middle_node,right=of first_step] (second_step) {b};
  \node[middle_node,right=of second_step] (third_step) {c}; 
  \node[end_node, right=of third_step] (stop) {\textbf{stop}};

  % lines
  \draw[ar] (start) -- (first_step);
  % here i don know how to bend this arrow to the same box
  \draw[ar] (first_step.80) -- (first_step.180);
  \draw[ar] (first_step) -- (second_step);
  \draw[ar] (second_step) -- (third_step);
  \draw[ar] (third_step) -- (stop);
\end{tikzpicture}
\end{document}

Answer 1

You can use a to-path with the option loop above or, more explicitly, with the options out=75, in=105, loop or similar:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[
node distance=1.5cm and 1cm,
ar/.style={->,>=latex},
middle_node/.style={
  draw,
  text width=1.5cm,
  minimum height=0.75cm,
  align=center
  },
end_node/.style={
  draw,
  text width=1cm,
  minimum height=0.55cm,
  align=center
  }
]

  % nodes
  \node[end_node] (start) {\textbf{start}};
  \node[middle_node,right=of start] (first_step) {a};
  \node[middle_node,right=of first_step] (second_step) {b};
  \node[middle_node,right=of second_step] (third_step) {c}; 
  \node[end_node, right=of third_step] (stop) {\textbf{stop}};

  % lines
  \draw[ar] (start) -- (first_step);
  % here i don know how to bend this arrow to the same box
  \draw[ar] (first_step) to[loop above] ();
  \draw[ar] (first_step) -- (second_step);
  \draw[ar] (second_step) to[out=75, in=105, loop] ();
  \draw[ar] (second_step) -- (third_step);
  \draw[ar] (third_step) -- (stop);
\end{tikzpicture}
\end{document}

---

If you want to start and end the loop exactly on the edges of the node, you should use the north east and north west anchors and perhaps use the looseness option to adjust how the loop is bent:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[
node distance=1.5cm and 1cm,
ar/.style={->,>=latex},
middle_node/.style={
  draw,
  text width=1.5cm,
  minimum height=0.75cm,
  align=center
  },
end_node/.style={
  draw,
  text width=1cm,
  minimum height=0.55cm,
  align=center
  }
]

  % nodes
  \node[end_node] (start) {\textbf{start}};
  \node[middle_node,right=of start] (first_step) {a};
  \node[middle_node,right=of first_step] (second_step) {b};
  \node[middle_node,right=of second_step] (third_step) {c}; 
  \node[end_node, right=of third_step] (stop) {\textbf{stop}};

  % lines
  \draw[ar] (start) -- (first_step);
  % here i don know how to bend this arrow to the same box
  \draw[ar] (first_step.north east) to[in=110, out=70, looseness=2] (first_step.north west);
  \draw[ar] (first_step) -- (second_step);
  \draw[ar] (second_step) -- (third_step);
  \draw[ar] (third_step) -- (stop);
\end{tikzpicture}
\end{document}